1. Field of the Invention
The invention relates to valves for implantable medical devices and more particularly to adjustable valve mechanisms which resist unintentional performance setting changes.
2. Description of the Related Art
There are a number of treatments for medical conditions which require fluid to be removed from an organ or tissue of a patient. One such condition is hydrocephalus, where cerebrospinal fluid abnormally accumulates in the skull faster than it is withdrawn by the body. The excessive build-up of cerebrospinal fluid compresses brain tissues, which eventually leads to brain damage.
Hydrocephalus is commonly treated by implanting a shunt in fluid communication with a ventricle within the brain to withdraw cerebrospinal fluid at a desired rate. Typically, the rate of withdrawal of cerebrospinal fluid is controlled by a valve having different pressure settings which a clinician adjusts pre-operatively. A number of shunt valves can be noninvasively changed after implantation, such as the Codman® Hakim® programmable valve which is currently commercially available from Codman & Shurtleff, Inc. of Raynham, Mass. Other adjustable valves include the Strata™ valve from Medtronic Neurosurgery, the ProGAV™ valve manufactured by Christoph Meithke GMBH and distributed by Aesculap AG, and the Sophy™ and Polaris™ valves available from Sophysa USA Inc. All of these valves utilize magnets to adjust valve pressure settings. To differing degrees, these valves are not optimal regarding resistance to unintentional setting changes, ease of use in achieving the desired valve setting, and detection of actual valve setting.
Magnetic resonance imaging, also referred to as MRI, is an increasingly common procedure for examining one or more regions of a patient. MRI provides better contrast between tissue types than computed tomography and utilizes powerful magnetic fields instead of potentially harmful x-rays. While magnetic exposure levels from first generation MRI systems were typically up to 1.5 Tesla, newer MRI machines routinely use 3.0 Tesla. As recognized by McCusker et al. in U.S. Pat. No. 7,390,310, for example, such strong magnetic fields can interfere with implanted devices including shunt valves.
It is therefore desirable to have easy-to-use implantable valves capable of withstanding strong magnetic fields of at least 3.0 Tesla and which resist unintended changes to valve settings.